[media] usbtv: improve a comment

Patrick Keshishian improved the explanation of the protocol when porting
the driver to OpenBSD. Given it's a reverse engineering one and there's
no documetnation it might be helpful to whoever hacks on the driver.

Signed-off-by: Patrick Keshishian <sidster@boxsoft.com>
Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>